The present invention relates to an optical waveguide fiber for transmitting light wave energy in a single mode.
Single-mode optical waveguide fibers have been developed that achieve transmission losses as low as 0.5 dB/km and 0.2 dB/km at wavelengths of 1300 nm and 1550 nm, respectively. Because of their low loss and because of the high bandwidths generally attributed to single-mode fibers, they are attractive as potential long distance transmission lines. However, their potentially high bandwidth can be achieved only if the design is optimized so that the total dispersion D.sub.t for the HE.sub.11 mode is equal to zero or as near as possible to zero at the operating wavelength.
In single-mode waveguides the total dispersion is governed by the material dispersion D.sub.m and the waveguide dispersion D.sub.w. For a given fiber composition the material dispersion varies as a function of wavelength. For example, the material dispersion versus wavelength curve passes through zero dispersion at a wavelength near 1280 nm for high silica content fibers. Single-mode fibers can be designed which exhibit zero total dispersion at any wavelength in a range of wavelengths above that wavelength at which the material dispersion curve passes through zero dispersion. This can be achieved by tailoring the waveguide dispersion to balance out the material dispersion at some specified wavelength which is selected because of low fiber attenuation and/or availability of light sources. The waveguide dispersion can be tailored by varying the core radius a, the core index profile or the core-cladding relative index difference .DELTA.. The term .DELTA. is defined by the equation .DELTA.=(n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2 wherein n.sub.1 is the peak refractive index of the core and n.sub.2 is the cladding refractive index. Techniques for tailoring the zero dispersion wavelength are taught in the article by U. C. Paek et al. entitled "Dispersionless Single-Mode Light Guides With .alpha. Index Profiles", The Bell System Technical Journal, Volume 60, No. 5, May-June 1981, pp. 583-598 and the article by L. G. Cohen entitled "Tailoring Zero Chromatic Dispersion Into The 1.5-1.6 .mu.m Low-Loss Spectral Region of Single-Mode Fibers", Electronics Letters, Volume 15, No. 12, June 7, 1979, pp. 134-135.
Whereas the designs taught in the aforementioned Paek et al. and Cohen et al. articles can result in a tailoring of the zero dispersion wavelength, they adversely affect other parameters. To achieve lowest system loss, there must be optimization of parameters such as spot size w.sub.o and the ratio w.sub.o /a, which determine splice loss and microbend loss, respectively. Also, work done on step-index single-mode waveguides having a .DELTA. of about 0.3% indicates that such a value of .DELTA. may be too low insofar as microbend loss is concerned. For conventional fibers having step-index or .alpha.-type core index profiles and having .DELTA.-values greater than about 0.3%, it is difficult to meet the requirement that the zero dispersion wavelength .lambda..sub.o be quite close, i.e., within 5 nm, to the laser source wavelength when the source wavelength is chosen to be about 1300 nm in order to reduce the loss contribution due to OH absorption which peaks at 1380 nm.
The Paek et al. publication states that as the wavelength gets longer, the guide radius must get smaller and that at longer wavelengths a much larger amount of material dispersion must be compensated for by waveguide dispersion. This requires greater precision in the waveguide parameters than when the guide is designed to operate at the zero of material dispersion. If the waveguide radius is made too small in order to balance out material dispersion, microbending losses become unacceptably high.
The W-type waveguide disclosed in U.S. Pat. No. 3,997,241 issued to S. Nishida et al. offers an additional parameter which can be varied in order to vary the waveguide dispersion. This fiber comprises a core having a uniform, relatively high refractive index n.sub.1 surrounded by an inner cladding layer having a relatively low refractive index qn.sub.1 and an outer cladding layer having an intermediate value of refractive index pn.sub.1. Since this design results in an increase of V.sub.c to a value calculated to be 3.8327, it enables light to be propagated in a single mode through a core having a radius greater than that which would be permitted in conventional step index waveguides. The normalized frequency V is expressed by the formula: ##EQU1## The term V.sub.c designates the single-mode cutoff value of V. Also, bending losses are reduced by the Nishida design. This design can achieve a total dispersion that is zero or near zero over a broad range of wavelengths, but in order to achieve such broad band operation, the intermediate layer index qn.sub.1 should be relatively low and the outer cladding index pn.sub.1 should be relatively close to the core index. In accordance with the teachings of the Nishida et al. patent, the quantity (n-pn)/(n-qn) should be less than 0.1. Such a small ratio of (n-pn)/(n-qn) causes manufacturing tolerance to be critical, and slight changes in the refractive index of a layer can greatly affect the slope of the waveguide dispersion curve. As the slope of the waveguide dispersion curve varies from its design value, the width of the wavelength range at which low dispersion operation can be achieved is correspondingly reduced.
The optical fiber of the Nishida et al. patent has a lower value of normalized frequency V.sub.1 ' below which single-mode propagation does not exist. As shown in FIG. 2 of that patent, single-mode propagation occurs in that range of normalized frequency between V.sub.1 ' and V.sub.2 '. Thus, as the index pn.sub.1 of the outer cladding is increased in order to satisfy the preferred relationship for the quantity (n-pn)/(n-qn), the V-value range over which single-mode operation is practical becomes small, again making the design sensitive to manufacturing tolerances.